JP3944578B2 - Strain and AE measuring device using optical fiber sensor - Google Patents

Strain and AE measuring device using optical fiber sensor Download PDF

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JP3944578B2
JP3944578B2 JP2003172321A JP2003172321A JP3944578B2 JP 3944578 B2 JP3944578 B2 JP 3944578B2 JP 2003172321 A JP2003172321 A JP 2003172321A JP 2003172321 A JP2003172321 A JP 2003172321A JP 3944578 B2 JP3944578 B2 JP 3944578B2
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Prior art keywords
strain
filter
change
reflected light
wavelength
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JP2005009937A (en
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浩 津田
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独立行政法人産業技術総合研究所
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    • GPHYSICS
    • G01MEASURING; TESTING
    • G01LMEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00Measuring force or stress, in general
    • G01L1/24Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infra-red, visible light, ultra-violet
    • G01L1/242Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infra-red, visible light, ultra-violet the material being an optical fibre
    • G01L1/246Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infra-red, visible light, ultra-violet the material being an optical fibre using integrated gratings, e.g. Bragg gratings
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01BMEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00Measuring arrangements characterised by the use of optical means
    • G01B11/16Measuring arrangements characterised by the use of optical means for measuring the deformation in a solid, e.g. optical strain gauge
    • G01B11/165Measuring arrangements characterised by the use of optical means for measuring the deformation in a solid, e.g. optical strain gauge by means of a grating deformed by the object

Description

[0001]
BACKGROUND OF THE INVENTION
In the present invention, a fiber Bragg grating (hereinafter referred to as “FBG”) sensor is used to detect a change in strain, and acoustic wave emission (acoustic emission, hereinafter referred to as “AE” associated with occurrence of microscopic damage of a material / structure). ").) Is detected.
[0002]
The present invention can be applied when an acoustic wave is generated using a piezoelectric element to evaluate the soundness of a structure, and further when a high-speed strain change due to an impact load is detected.
[0003]
That is, the present invention can be applied to simultaneously measuring strain for examining the load of a material or a structure and a damage state and AE associated with occurrence of microscopic destruction with one FBG sensor. The present invention is expected to be used for soundness evaluation of automobiles, aircraft, bridges, buildings, and the like.
[0004]
[Prior art]
Conventionally, a technology has been used in which AE is detected using a piezoelectric element, and impact load is detected using a strain gauge.
[0005]
Further, a method has been proposed in the United States in which a reflected wave from an FBG sensor is passed through an FBG having a Bragg wavelength substantially equal to the Bragg wavelength of the FBG sensor and AE is detected from the transmitted light (see Non-Patent Document 1).
[0006]
Furthermore, regarding the Bragg wavelength change of the FBG sensor, conventionally, the reflected wave wavelength from the FBG sensor is measured by an optical spectrum analyzer to measure the distortion.
[0007]
[Non-Patent Document 1]
I. Perez, H.-L.Cui and E. Udd, 2001 SPIE, Vol. 4328, p.209-215
[0008]
[Problems to be solved by the invention]
However, the conventional technique using a piezoelectric element for detecting AE has a drawback that it is affected by electromagnetic interference because measurement parameters are directly converted into electric signals for measurement.
[0009]
In addition, the FBG sensor does not suffer from electromagnetic interference because it converts the measurement parameter into an optical signal, but the detected waveform cannot always reproduce the original waveform of AE, and the waveform may be distorted.
[0010]
In addition, the technology that measures the Bragg wavelength change of an FBG sensor using an optical spectrum analyzer usually has a sampling rate of about 1 sampling per second. It is impossible to detect and detect minute changes in strain with a frequency characteristic of 100 kHz. For this reason, there is a problem that a high-speed strain change cannot be detected following up.
[0011]
An object of the present invention is to solve such a conventional problem, and an object of the present invention is to realize an optical fiber strain sensor having the following characteristics.
(1) High-speed strain change due to AE or impact load can be detected accurately when the FBG sensor is detected. (2) By changing the reflection characteristics of the filter, a small strain can be obtained with a single FBG sensor. It is possible to detect a wide range of strain changes from the AE that is a change to the impact load where a large strain change occurs.
(3) Since the FBG sensor converts measurement parameters into optical signals, it is not subject to electromagnetic interference.
(4) A single FBG sensor measures both strain and AE, and reduces the number of sensors in strain and AE measurement, which are important for evaluating the soundness of materials and structures.
[0012]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention provides an FBG sensor made of an optical fiber in which FBG is written and attached to a subject, a broadband light source for making broadband wavelength light incident on the FBG sensor, and transmitted from the FBG sensor. A strain sensor using an optical fiber sensor, and a strain measurement filter and an AE detection filter for reflecting or transmitting the reflected light branched by the coupler, respectively. The strain measurement filter and the AE detection filter have different transmittances corresponding to each of the two types of wavelengths, and the transmission of the strain measurement filter and the AE detection filter is different. The intensity of light or reflected light changes due to the change in Bragg wavelength, and these are converted into electrical signals by a photoelectric converter to detect strain change and AE simultaneously. We provide a measuring device for characteristic optical fiber strain and AE.
[0013]
Based on the information on the distortion change obtained by converting the photoelectric signal into the electrical signal by the photoelectric converter, the wavelength band in which the transmittance of the AE detection filter for the AE detection changes is controlled, and the AE detection The AE can be measured from the transmitted light intensity of the filter, the reflected light intensity, or the difference between the transmitted light intensity and the reflected light intensity.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of a strain and AE measuring apparatus using an optical fiber sensor according to the present invention will be described below with reference to the drawings based on examples.
[0015]
(FBG operation principle)
Prior to the description of the present invention, the principle of FBG, which is the basis of the present invention, will be described with reference to FIG. Light from the broadband light source enters the optical circulator terminal (1) through the FBG sensor (optical fiber into which the FBG is written) connected to terminal (2).
[0016]
From the FBG sensor, a narrow-band light component having a central wavelength (referred to as “Bragg wavelength” in this specification) λ B given by twice the product of the refractive index n and the refractive index change interval Λ. Are reflected, and other light components pass through the FBG sensor. As shown in the figure, the optical circulator sends the reflected light from the FBG sensor connected to the terminal (2) to the terminal (3).
[0017]
FIG. 2 is a diagram showing the relationship between the Bragg wavelength and the strain received by the FBG sensor. When the FBG sensor is distorted, the distance between the refractive index changes and the refractive index change. Now, when the FBG sensor is subjected to a strain ε in the fiber axis direction, the change [Delta] [lambda] B of the Bragg wavelength lambda B is a constant temperature conditions, it is known that given by the following equation 1.
[0018]
[Expression 1]
[0019]
Here, p e is a photoelastic constant (= 0.213), and ε is an optical fiber axial strain that the FBG receives. Therefore, the Bragg wavelength moves to the long wavelength side when the FBG sensor is subjected to tensile strain, and to the short wavelength side when it is subjected to compressive strain. For example, when an FBG sensor with a Bragg wavelength of 1550 nm undergoes a strain change of 1 × 10 −6 , the Bragg wavelength changes (shifts) by 1.2 pm. In short, the center wavelength of the reflected wave from the FBG sensor fluctuates in proportion to the strain change experienced by the FBG.
[0020]
(Features of the present invention)
[0021]
The features of the present invention will be described below. In the present invention, in order to measure the change in the Bragg wavelength at high speed, the reflected light from the FBG sensor is passed through filters having different transmittances according to the wavelength, and the change in the Bragg wavelength is converted into a change in light intensity.
[0022]
Then, the reflected light from the FBG sensor is split into two fibers via a 1x2 coupler via an optical circulator, and each of them is used as a strain measurement filter and an AE detection filter with different transmittances according to two wavelengths. Pass through. The transmitted light and reflected light of both filters change in intensity due to the change in Bragg wavelength. By detecting these with a photoelectric converter, strain change and AE can be detected simultaneously.
[0023]
The strain measurement filter is assumed to have a characteristic that the transmittance changes in a wide wavelength range as compared with the AE detection filter. An FBG sensor with a Bragg wavelength of 1550 nm without strain produces a wavelength shift of 1.2 pm per 1 × 10 −6 strain, so if the object to be inspected is expected to be strained up to ± 1%, As a result, a Bragg wavelength change of ± 12 nm occurs. Therefore, the strain change can be measured by a filter whose transmittance varies in the wavelength range of 1538 to 1562 nm.
[0024]
The present inventor has already proposed in Japanese Patent Application No. 2002-340197 a configuration for detecting AE from this measurement system using FBG as an AE detection filter. For AE detection, it is necessary that the reflection wavelength band from the FBG sensor is in the transmittance change wavelength band of the AE detection filter.
[0025]
Since the transmittance change of the filter for AE detection is limited to a very narrow wavelength range of about 0.4 nm or less, the Bragg wavelength of the reflected light from the sensor fluctuates greatly when a large strain change occurs, and AE detection May be out of the transmittance change wavelength range of the filter. For this reason, the AE detection filter is preferably a tunable filter whose transmittance change wavelength band changes in accordance with the strain that the inspection object receives.
[0026]
Strain is measured by converting the transmitted light and reflected light of the strain measurement filter into electrical signals by a photoelectric converter. Based on this distortion information, the operating wavelength range (wavelength band in which the transmittance changes) of the AE detection tunable filter is controlled. The AE can be measured from the transmitted light or reflected light intensity of the tunable filter, or the difference between the transmitted light intensity and the reflected light intensity.
[0027]
With this system, strain is measured from the signal passed through the strain measurement filter, and AE is measured from the signal passed through the AE detection filter. Examples 1 to 3 of the present invention will be described below with reference to the drawings.
[0028]
Example 1
FIG. 3 is a diagram for explaining a strain and AE measuring apparatus using the optical fiber sensor according to the present invention. In FIG. 3, light from a broadband light source enters an FBG sensor via an optical circulator. The FBG sensor is fixed to the object to be measured.
[0029]
Reflected light from the FBG sensor is passed through a 1 × 2 coupler via an optical circulator. The 1 × 2 coupler branches the reflected light from the FBG sensor into two optical fibers. One optical fiber is connected to a strain measurement filter, and the other is connected to a tunable filter for AE detection.
[0030]
The transmitted light and reflected light of each filter are connected to a photoelectric converter, and the intensity of each light is converted into an electrical signal. The reflected light of the filter can be taken out by attaching an optical circulator in front of the filter. Strain can be measured from the transmitted light intensity and reflected light intensity of the strain measurement filter.
[0031]
The photoelectric converter Sst is connected to the tunable filter control unit. The tunable filter control unit evaluates the Bragg wavelength moved due to the strain change, and sends a signal for controlling the operating wavelength range (wavelength range in which the transmittance changes) of the tunable filter to the tunable filter.
[0032]
FIG. 4 is a diagram showing the principle of strain measurement using a strain measurement filter. In FIG. 4, when the FBG sensor is distorted, the Bragg wavelength changes. The transmitted light obtained through the reflected light from the FBG sensor through the filter whose transmittance varies with wavelength, and the reflected light intensity vary depending on the position of the Bragg wavelength.
[0033]
For example, as shown in the upper diagram of FIG. 4, when the reflected light from the FBG sensor is passed through a filter whose transmittance decreases as the wavelength increases, the FBG sensor is subjected to compressive strain (the Bragg wavelength is short). (Shifted to the wavelength side), the transmitted light intensity of the filter increases.
[0034]
Also, as shown in the lower diagram of FIG. 4, when the tensile strain is applied (the Bragg wavelength is shifted to the longer wavelength side), the transmitted light intensity of the filter is lowered. The Bragg wavelength change can be evaluated as the electric signal intensity by converting the transmitted light intensity change into an electric signal by the photoelectric converter.
[0035]
As for the reflected light intensity, the signal intensity changes with the Bragg wavelength change according to the same principle from the relationship of reflectance = 1−transmittance. That is, the transmitted light intensity and the reflected light intensity of the filter change due to strain. However, the intensity received by the photoelectric converter changes each time the optical fiber connector is connected. This is caused by misalignment of the connector connecting portion. For this reason, distortion cannot be quantitatively evaluated by transmitted light or reflected light intensity alone. Strain can be quantitatively evaluated from the value obtained by dividing the difference between the transmitted light intensity and the reflected light intensity by the sum of both intensities.
[0036]
The upper diagram in FIG. 5 is a diagram for explaining the principle of AE detection by the FBG sensor. Since the strain change due to AE is minute, in order to detect AE by the FBG sensor, a filter with a narrow wavelength band in which the transmittance is changed compared to strain measurement is required. The Bragg wavelength of the reflected light from the FBG sensor is slightly changed by AE.
[0037]
This Bragg wavelength change is converted into an intensity change by passing it through a filter having a narrow-band transmittance change. For example, as shown in FIG. 5, when there is no AE, reflected light of the Bragg wavelength λ s is returned from the FBG sensor, and the center wavelength of the transmittance change of the AE detection filter is λ F. The Bragg wavelength changes when the FBG sensor undergoes strain change due to AE. Bragg wavelength changes compressed, and respectively the tensile strain lambda s', and lambda s' on '.
[0038]
The transmitted light intensity of the filter changes in proportion to the area represented by diagonal lines due to the strain change due to AE. Therefore, the output of the photoelectric converter that converts the transmitted light intensity of the filter into an electric signal due to the strain change due to AE is as shown in the lower diagram of FIG. Regarding the reflected light intensity, the reflected light intensity changes when AE is detected in the same manner from the relationship of reflectance = 1−transmittance. Also, the difference between the transmitted light and reflected light signal intensity varies with AE on the same principle.
[0039]
As a filter for detecting AE, there are a dielectric multilayer filter and FBG. In the example of this figure, the band-pass filter is used as the AE detection filter, but a low-pass or high-pass filter may be used.
[0040]
FIG. 6 is a diagram illustrating the shift of the tunable filter operating wavelength band in accordance with the strain change. The filter for detecting AE has a wavelength region in which the transmittance is about 0.4 nm.
[0041]
For this reason, when the Bragg wavelength shifts by several nm due to a large strain change, the wavelength range of the reflected light of the FBG sensor deviates from the wavelength range where the transmittance of the filter for AE detection changes. For this reason, in order to perform AE measurement, it is necessary to change the operating wavelength range of the filter for AE measurement according to the strain change measured by the filter for strain measurement.
[0042]
The tunable filter can change the operating wavelength range by an external control signal. By using a tunable filter as the filter for AE measurement, the operating wavelength range of the filter for AE measurement is controlled according to the strain received by the FBG sensor. At this time, distortion information evaluated from a value obtained by dividing the difference between the transmitted light intensity and the reflected light intensity of the strain measurement filter by the sum of both intensities is used for controlling the operating wavelength region of the tunable filter.
[0043]
(Example 2)
FIG. 7 shows a second embodiment of the present invention, which is configured to enable simultaneous multipoint strain and AE measurement by a plurality of FBG sensors. We show a device that measures strain and AE simultaneously at multiple points by arranging FBG sensors with different Bragg wavelengths in series. The reflected light from the FBG sensor array is separated into signals from the respective FBG sensors by an optical demultiplexer and output. In FIG. 7, the optical circulator before the filter and the extraction of the reflected light from the filter are not shown in order to simplify the drawing.
[0044]
FIG. 8 shows a third embodiment of the present invention, and shows a configuration that enables measurement of strain and AE at a specific location by a plurality of FBG sensors. This shows a device that measures strain and AE at the location where a specific FBG sensor is attached by arranging FBG sensors with different Bragg wavelengths in series.
[0045]
Only the reflected light component from the desired FBG sensor is extracted through the tunable filter through the reflected light from the FBG sensor array from the optical circulator. The operating wavelength band of the strain measurement filter is also changed in conjunction with it. In FIG. 8, the optical circulator before the filter and the extraction of the reflected light from the filter are omitted to simplify the drawing.
[0046]
As described above, the embodiments of the strain and AE measuring apparatus using the optical fiber sensor according to the present invention have been described based on the examples. However, the present invention is not limited to such examples, and It goes without saying that there are various embodiments within the scope of the technical matter described.
[0047]
【The invention's effect】
According to the strain and AE measuring device using the optical fiber sensor having the above configuration, both the strain and the AE can be simultaneously measured by one sensor using the FBG sensor.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a principle diagram of FBG.
FIG. 2 is a diagram illustrating the relationship between Bragg wavelength and strain.
FIG. 3 is a diagram illustrating Example 1 of the present invention.
FIG. 4 is a diagram illustrating the operation of the first embodiment.
FIG. 5 is a diagram illustrating the operation of the first embodiment.
FIG. 6 is a diagram for explaining the operation of the first embodiment.
FIG. 7 is a diagram illustrating Example 2 of the present invention.
FIG. 8 is a diagram illustrating Example 3 of the present invention.

Claims (2)

  1. An FBG sensor comprising an optical fiber in which FBG is written and attached to a subject; a broadband light source for allowing broadband wavelength light to enter the FBG sensor; a coupler for branching reflected light transmitted from the FBG sensor; and the coupler A strain and AE measurement device using an optical fiber sensor comprising a strain measurement filter and an AE detection filter that respectively reflect or transmit reflected light branched by
    The strain measurement filter and the AE detection filter have different transmittances corresponding to the two types of wavelengths,
    The transmitted light or reflected light of the strain measurement filter and the AE detection filter changes in intensity due to the change of the Bragg wavelength, and these are converted into electrical signals by a photoelectric converter to detect strain change and AE simultaneously. Optical fiber strain and AE measuring device characterized by this.
  2. Based on the information related to the distortion change obtained by converting to an electrical signal by the photoelectric converter, the wavelength band in which the transmittance of the AE detection filter for AE detection changes is controlled, and the AE detection 2. The strain and AE measuring apparatus using an optical fiber sensor according to claim 1, wherein the AE can be measured from the transmitted light intensity of the filter, the reflected light intensity, or the difference between the transmitted light intensity and the reflected light intensity.
JP2003172321A 2003-06-17 2003-06-17 Strain and AE measuring device using optical fiber sensor Active JP3944578B2 (en)

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JP2003172321A JP3944578B2 (en) 2003-06-17 2003-06-17 Strain and AE measuring device using optical fiber sensor
PCT/JP2004/008315 WO2004113830A1 (en) 2003-06-17 2004-06-14 Strain and ae measurement device using optical fiber sensor

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